Paint compositions containing light reflective flakes and colorants are used for protective and decorative coatings, for example, on automobile bodies. Such coatings accentuate the styling lines and contoured surfaces and are characterized by what is known as geometric metamerism. Geometric metamerism, also referred to as goniochromatism or "flop", is the ability of a paint film to exhibit subtle variations in color and lightness depending upon the direction from which it is viewed. This effect is produced by metal or other reflective flakes such as mica incorporated into the film at varying depths. The aesthetics of such decorative and protective coatings are further influenced by the sharpness of the image outlines. This characteristic is known in the paint industry as "distinctness of image" (DOI), or simply "gloss".
Acceptable appearance of a metallic pigmented coating further depends on the correct covering power, or optical density, provided by the metal particles. Acceptable covering power is usually defined as that which matches the appearance of a reference standard. If no such standard exists, the coating should exhibit a covering power sufficient to obscure the substrate onto which it is protectively bonded.
Typical prior art paint coatings containing reflective flakes, sometimes referred to as base coats, which are said to have a high degree of geometric metamerism are formed by the spray application of a single layer as described, e.g., in U.S. Pat. No. 4,681,811, or by the application of dual layers of identical composition as described, e.g., in U.S. Pat. No. 4,603,064 and U.S. Pat. No. 4,719,132.
Numerous attempts have been made to improve the appearance of protective and decorative paint coatings having reflective metallic flakes for use on substrates, such as automobile panels. Disclosures in the following patents are representative: U.S. Pat. Nos. 4,725,640; 4,213,886; 4,199,489; 4,611,026; 4,359,504; 3,932,349; 4,484,951; and 4,403,003. The disclosures in the above-listed patents are hereby incorporated by reference in their entirety.
These prior attempts have all had one or more disadvantages such as an undesirably high concentration of metallic flakes, the need for an added expensive ingredient, undesirably thick paint layers, inadequate flop or inadequate covering power.
An advantage of larger flake size in coating compositions containing metal flakes is suggested in U.S. Pat. No. 3,692,731, which indicates in column 1 that greater sparkle can be provided if aluminum is used in smaller amounts in the form of large insoluble resin supported flakes of increased planarity so that a greater proportion of the flakes reflect light in unison, creating more widely separated areas having increased sparkle.
However, as is indicated in Paint and Surface Coatings: Theory and Practice, Chapter 10, entitled "Automotive Paints", p. 466, there has been a bias against formulating automotive paints with coarse flakes (sizes up to about 30 .mu.m) for use in basecoats due to poor opacity and the problem of flake protrusion through the basecoat surface which can result in an unacceptable seedy appearance.
Thus, the problem facing the art has been to provide a paint coating having the advantages of large flakes yet having good opacity and improved appearance.